1. Field
This invention pertains to methods to remove chemicals/pharmaceuticals from wastewater entering wastewater treatment plants employing biological reduction. In particular it pertains to a sulfurous acid/alkalinization pre-treatment method for wastewaters to condition and remove a number of chemicals/pharmaceuticals and heavy metals before the recovered wastewater undergoes further biological reduction via anaerobic and aerobic digestion.
As used herein, the term chemicals/pharmaceuticals are defined as any chemical used for the diagnosis, treatment (cure/mitigation), alteration, or prevention of disease, health condition, or structure/function of the body; and other chemicals entering wastewater streams, such as perchlorates and other fertilizer compounds. The term shall also refer to “pharmaceutical,” pharmaceutically active compounds” (PhAC), and “pharmaceuticals and personal care products” (PPCP), which have wide-ranging physical and chemical properties.
2. State of the Art
The presence of human pharmaceutical compounds in surface waters is an emerging environmental issue. Many types of pharmaceuticals are not affected by sewage treatment processes, while others are amenable to some degree of degradation. While water purification techniques such as granular activated carbon could potentially remove these pollutants from wastewater streams, the high costs, their intertwined presence with other solid and liquid components, and handling difficulties involved in these complex aqueous systems require more attention be given to the potential for the optimization of current treatment processes in order to reduce environmental contamination.
In the article entitled “Human Pharmaceuticals in Wastewater Treatment Processes Posted on: Sunday, 31 Jul. 2005, 03:00 CDT, http://www.redorbit.com/news/science/192358/human_pharmaceutical . . . , discusses the current methods for addressing pharmaceutical and chemical reduction:
“A. Biodegradation
There is an obvious potential for biological degradation (aerobic/anaerobic by micro-organisms) of drug substances leading to a reduction of the parent compounds and/or their metabolites during wastewater treatment. Some biodegradation may also occur during in-pipe transport to the STP, but most will probably occur in the secondary stage of treatment when the compound is exposed to large concentrations of micro-organisms. Biodegradation tests can be performed following test protocols such as the closed bottle test (OECD 301D) or the Zahn-Wellens test (OECD 302B). In general, these tests are carried out with several hundred milligrams of a substance as the carbon source. Therefore, they give answers for only fairly extreme conditions, which, despite their intention, simulate only the maximum potential and not the most probable environmental outcome. Therefore, conclusions on the degradability of drugs in STPs [sewage treatment plants] from these tests are of limited value and further research is necessary.
Al-Ahmad et al. assessed the biodegradability of the clinically important antibiotics cefotiam, ciprofloxacin, meropenem, penicillin G, and sulfamethoxazole using the closed bottle test (CBT). None of the test compounds met the criteria for ready biodegradability. Of all the compounds studied, only penicillin G was found to be biodegradable to some degree, with approximately 27% being removed after 28 days. Even when the test was prolonged to 40 days, the removal rate was only increased to 35% indicating the compound was relatively stable.
Kmmerer and Al-Ahmad used the CBT and the modified ZahnWellens test (ZWT) to examine the biodegradability of the widely used antitumor agents 5-fluorouracil, cytarabine, and gemcitabine. 5-Fluorouracil was not biodegradable in either of these tests. Gemcitabine was biodegraded by 42% in the CBT, but prolonging the test period to 40 days only improved this to 45%. Cytarabine was also partially biodegraded in the CBT (50%). In the ZWT, the biodegradation of gemcitabine was also 50% but only after an adaptation period of 20 days, which is not normally included in such tests. Prolonging the test to 40 days improved the degree of biodegradation to 80%, and in the ZWT the biodegradability was over 95%.
Henschel et al. investigated the biodegradability of paracetamol and methotrexate and the two drug metabolites salicylic acid and clofibric acid. Their results were in agreement with other studies and demonstrated that salicylic acid and (to a lesser extent) paracetamol were biodegradable, whereas clofibric acid and methotrexate were not.
Kmmerer studied the biodegradability of three clinically important antibiotics (ciprofloxacin, ofloxacin, and metronidazole) and found none of the compounds were biodegraded. As a consequence the genotoxicity of these compounds (as measured by the SOS chromotest) remained unaffected after treatment. A more comprehensive review of antibiotics in the environment is available in Hirsch et al. This article describes the analysis of various water samples for 18 antibiotic substances, from several groups, including macrolid antibiotics, sulfonamides, penicillins, and tetracyclines. Both STP effluents and surface-water samples were frequently contaminated with sulfamethoxazole and roxithromycin (a degradation product of erythromycin) at concentrations up to 6 g L−1. The highest concentrations detected for tetracyclines and penicillins were 50 and 20 nL−1, respectively. Except for two sites, no contamination by antibiotics was detected from a large number of groundwater samples that were taken from agricultural areas in Germany. This suggests that contamination of groundwater by antibiotics from veterinary applications is relatively minor. Other drugs that have been investigated for their biodegradability include ifosfamide and cyclophosphamide. Both of these compounds exhibited poor biodegradability in the CBT and the ZWT as well as in laboratory-scale activated sludge plants.
Degradation may also occur during bank filtration, if it is used. Heberer et al. found clofibric acid, phenazone, propyphenazone, diclofenac, ibuprofen, and fenofibrate, and two metabolites, jV-methylphenacetin (probably originating from phenacetin) and also a derivative of clofibric acid at concentrations up to the micrograms per liter level in groundwater samples taken from beneath a drinking-water treatment plant. These contaminants were found to leach from the neighboring sewage contaminated surface water by bank nitration through the subsoil.
Molecules with long, highly branched side chains are generally less amenable to biodegradation than unbranched compounds with shorter side chains. Unsaturated aliphatic compounds are generally more accessible to biodegradation than saturated analogues or aromatic compounds with complicated aromatic ring structures and sulfate or halogen groups. Examples of the latter are the x-ray contrast media. Since these compounds are exclusively utilized in human medicine, contaminated STP effluents are presumably the sole sources for these compounds in the aquatic environment. The occurrence of four iodinated x-ray contrast media (diatrizoate, iopamidol, iopromide, and iomeprol) in eight German STPs was examined by Ternes et al. These compounds were found to be ubiquitously distributed in the raw sewage and were not significantly degraded or absorbed during the sewage treatment processes and so remained in the aqueous phase. The concentrations of diatrizoate, iopromide, and iomeprol frequently exceeded 1 g L^sup −1^ in the raw sewages, and these were found at comparable concentrations in the final effluents, with the maximum concentration measured being 15 gL^sup −1^ for iopamidol.
A similar study by Steger-Hartmann et al. demonstrated that while these compounds are not readily biodegradable, iopromide was amenable to photo degradation. The resulting degradation product (5-amino-N,N′bis(2,3-dihydroxypropyl)-2,4,6-triiodo-N-methyliso-phthalamide) also exhibited a faster rate of photolysis than the parent compound and was further degraded in a test system simulating surface-water conditions. However, the predicted environmental concentration (PEC) in surface water was still high at 2 L^sup −1^.
Some degradation of iopamidol in activated sludge has also been observed with 85% being transformed into two metabolites. Degradation of the same compound in river water was even more significant, with a half-life of 3-1 days. However, for other, similar compounds such as diatrizoate the half-life was longer, suggesting there is potential for some compounds to reach rivers and lakes. Although of low toxicity, x-ray contrast media may contribute significantly to the absorbable organic halogen compound (AOX) load in receiving waters. This is of concern because of the high persistence, mobility, and potential of these substances to biotransform to toxic breakdown products.
It is also possible that the biota of a STP may gradually become acclimatized to certain chemicals and therefore may degrade them more effectively given time. For instance Zwiener et al. investigated the biological degradation of pharmaceutical residues (clofibric acid, ibuprofen, diclofenac). In this study both a pilot sewage plant and biofilm reactors operating under oxic and anoxic conditions were run as model systems for municipal sewage treatment, with synthetic sewage and pharmaceuticals in concentrations of 10 L^sup −1^. Clofibric acid displayed persistence in all cases. The pilot sewage plant and the anoxic biofilm reactor showed comparable results for diclofenac and ibuprofen, which both were partially degraded. A high degree of degradation was found for ibuprofen in the oxic biofilm reactor, which was attributed to adaptation of the biofilm to the residue. This effect has also been show to occur for other compounds, for example, nitrilotriacetic acid, where a period of acclimatization has been shown to be required before biodegradation can begin.102 In addition, the phenomenon of co-metabolism—the oxidation and degradation of nongrowth substrates by micro-organisms—is well documented.
B. Deconjugation
Pharmaceutical compounds are often metabolized in the liver, and as a consequence gluconoride and sulfate conjugates of the parent drugs are excreted. Conjugates of other organic compounds such as steroid hormones have been shown to be readily deconjugated in domestic wastewater and within STPs due to the large amounts of β-glucuridase enzyme present (produced by the fecal bacterium Escherichia coif). It seems probable that gluconoride and sulfate conjugates of drug compounds will be degraded by the same process. The effect will be to increase the excreted contribution of the active drugs to sewage and effluents.
C. Partitioning
Partitioning between the aqueous and organic biomass phases is a key component in determining the ultimate concentrations of organic pollutants. Compounds with high log K^sub ow^ values are known to sorb to sludge, while substances with lower values are more likely to stay in the aquatic phase, depending on the individual compound, 40 and substances sorbing to solids may also be remobilized if they are not strongly bound. It is also well known that bacterial, algal, and fungal cells are capable of adsorbing and accumulation organic pollutants. The activated sludge biomass is able to adsorb organic pollutants such as lindane, and adsorption of these compounds generally fits the Freundlich isotherm. There is a good correlation between compound adsorption and the octanol/water partition coefficient. However, since most drugs are soluble with low log K^sub ow^ and K^sub oc^ values, they exist primarily in the aqueous phase and transfer to sewage sludge is probably of only minor concern for the majority of compounds.
There are few studies in the literature detailing potential sorption interactions of drug compounds. Hua et al. studied the removal of chemical oxygen demand (COD), microorganisms, and selected pharmaceutical compounds by trickling wastewater through a sandy soil from the Rhine valley in glass columns. The sewage contained low concentrations of at least 10 different pharmaceuticals and x-ray media. Some of the compounds were removed by adsorption onto sand and/or biodegradation. The rate of removal varied from complete (e.g., ibuprofen and naproxen), to almost none, for several x-ray contrast media. Some of the compounds were removed as effectively by this method as by conventional sewage treatment.
Jones et al. estimated physicochemical values for the top 25 pharmaceuticals in England in 2000 using a computer model. Of the top 25 compounds, 16 had low predicted sorption potential and were thought unlikely to bind to sludge solids. Five compounds had medium sorption potential and two (quinine sulfate and mefenamic acid) were predicted to have a high capacity to bind to solids (no data were available for the remaining two compounds). Although this study indicated some removal to solids for mefenamic acid, it did not demonstrate that all would be removed and in fact this compound has been found to be present in sewage effluent. The concentrations of mefenamic acid in three sewage effluents as well as upstream and downstream of the effluent discharge point have been reported by Hilton and Thomas. The report does not, however, quote influent concentrations, so it is impossible to say how much was lost during treatment. For instance, if the concentration in the influent was double the concentration in the effluent, this would indicate a potentially high binding capacity (or biodegradation rate). A second paper by Soulet et al. indicates a high degree of variation in the removal of mefenamic acid depending on the STP studied. Some exhibited a high removal, while others showed almost none indicating the importance of design and operational factors and/or climatic conditions. This means a definitive conclusion cannot be reached with regard to the removal rates of this drug within STPs, other than that it is potentially highly dependent on plant design, wastewater characteristics, and, most importantly, the operational regime.
Bester studied the fate of the antimicrobial triclosan (2,4,4′-trichloro2′-hydroxyphenyl ether) in a German sewage treatment plant that processed 200,000 m^sup 3^ wastewater per day. The concentrations in the influent (˜1000 ng L^sup −1^) were compared to those in the effluent and the sludge, and a mass balance of the works was prepared. Thirty percent of the triclosan was found to sorb to the sludge with weak bonds, but only about 5% of the influent concentration was found in the effluent. Thus, most of the incoming material was not recovered as the parent compound, and the authors suggest that it is likely that it is transformed to other metabolites or unrecovered bound residues. This compares well with a study by Singer et al., who, during a field study, attributed the fate of triclosan in a wastewater treatment plant to be 79% biological degradation, 15% sorption to sludge, with 6% being discharged to the receiving surface water. Despite the high overall removal rate, the concentration i\n the wastewater effluents were in the range of 42-213 ng L^sup −1^, leading to concentrations of 11-98 ng L^sub −1^ in the receiving rivers.
A recent review of veterinary drugs by Tolls suggests that mechanisms other than hydrophobic partitioning play a significant role in sorption of animal (and potentially human) medications. A number of hydrophobicity independent mechanisms such as cation exchange, cation bridging at clay surfaces, surface complexation, and hydrogen bonding also appear to be involved. These processes are not accounted for by organic carbon normalization, suggesting that this data treatment is conceptually inappropriate and fails to describe the sorption behavior. In addition, some drug compounds may be anions at the pH values in STPs and the environment. This will lower the effective K^sub ow^ and decrease their sorption potential.
D. Removal During Sludge Treatment
Drugs may also be degraded during sewage treatment processes. Many pharmaceuticals are not thermally stable and so might be expected to break down during processes such as composting due to heat (as well as chemical and biodegradation). A study by Guerin investigated soil composting as an alternative to incineration for the treatment of a silty clay soil that had become contaminated with residues of Probenecid (an antigout drug) and Methaqualone (a barbiturate substitute no longer available due to harmful side effects). In pilot scale trials, Probenecid was reduced from 5100 mg kg^sup −1^ to <10 mg kg^sup −1^ within 20 weeks during mesophilic treatments. The study also confirmed that thermophilic composting was effective under field conditions. In the full-scale treatment, 180 tons of soil were composted. Initial concentrations of the major contaminants in the full-scale compost treatment facility for Probenecid and Methaqualone were 1160 mg kg^sup −1^ and 210 mg kg^sup −1^, respectively. Probenecid concentration reached the target level of 100 mg kg^sup −1^ in 6 weeks, and removal of Methaqualone to <100 mg kg^sup −1^ was achieved after 14 weeks. The study concluded that composting was effective in reducing soil concentrations of Probenecid and Methaqualone residues to acceptable values and hence is a technology that has potential application in the remediation of pharmaceutical contaminants in sludge/soil, although further testing using other drug compounds and soils would be necessary.
E. Photodegradation
Several pharmaceutical compounds have been shown to degrade due to the action of sunlight. The most extensively studied of these compounds is the analgesic/anti-inflammatory drug diclofenac, which has been shown to degrade in the aquatic environment due to ultraviolet (UV) light. Other compounds such as the topical antimycotic drugs naftifine, sulbentine, cloxiquin, tolnaftate, and chlorphenesin have also been shown to be light sensitive, and an overall elimination rate of 0.03 day^sup −1^ due to photochemical degradation was observed for triclosan in the epilimnion of Lake Greifensee by Singer et al.
Andreozzi et al. carried out a monitoring survey of STP effluents in Italy, France, Greece, and Sweden and found more than 20 individual pharmaceuticals. The photodegradation of six compounds (carbamazepine, diclofenac, clofibric acid, ofloxacin, sulfamethoxazole, and propranolol) was tested. Carbamazepine and clofibric acid were found to have the longest half-lives (of the order of 100 days at the most northerly areas sampled), whereas sulfamethoxazole, diclofenac, ofloxacin, and propranolol were found to undergo faster degradation with half-lives of 2.4, 5.0, 10.6, and 16.8 days, respectively. For almost all the studied compounds, except propranolol, the presence of nitrate ions in aqueous solutions resulted in a reduction of the measured half life. This effect may be ascribed to the formation of HO radicals due to photolysis of nitrate. The authors point out that besides pharmaceutical residues, other species targeted by OH radicals, such as naturally occurring organic constituents, are present in rivers and lakes. For this reason, the effect caused by nitrate on the degradation rates of the pharmaceuticals found in this study should be interpreted only as a tendency if no other organic molecules but the substrate are present in the test solution.
A more complex situation arose when humic acids were added to the solutions containing the pharmaceuticals. Humic acids are known to exert two opposite effects on the rate of photodegradation of organic molecules in water. Due to their capability to absorb UV radiation in a broad range of wavelengths, they can reduce the available energy for the organic molecules present in the solution, thus acting as an inner filter (thus decreasing photodegradation). At the same time, the molecules of humic acids submitted to UV irradiation are promoted to a transient, excited state, in which they may react with oxygen in the solution, forming reactive species as singlet oxygen, or react directly with other organic species, thus promoting their phototransformation. The overall effect of humic acids on the phototransformation rate of an organic substance will therefore depend on the balance between these two opposite contributions. In the study, humic acids were found to act as inner filters toward carbamazepine and diclofenac, but as photosensitizers toward sulfamethoxazole, clofibric acid, oflaxocin, and propranolol.
Buser et al. established that up to 90% of diclofenac entering a Swiss lake was degraded with a half-life of less than 1 h^sup −1^. Incubation of lake water, fortified with diclofenac, exhibited no reduction in the dark, suggesting minimal chemical and biological degradation. However, when the fortified water was exposed to sunlight, rapid degradation was observed that indicated that this was the result of photodegradation. The use of sewage lagoons may therefore increase the removal of light sensitive compounds as demonstrated by Kreuzinger et al., who showed that removal rates of diclofenac were only 14% with just activated sludge treatment, while after further polishing in a sewage lagoon concentrations decreased to below the limits of detection. Adsorption and biodegradation were ruled out as the cause of the decrease, as there was no developed/active sludge flock in the lagoon, leaving photodegradation as the most likely cause.
Latch et al. studied the photochemical fates of the histamine H2receptor antagonists cimetidine and ranitidine. Each displayed high rates of reaction with both single oxygen and hydroxyl radicals, with two transient oxidants being formed in sunlit natural waters. Ranitidine was degraded in direct photolysis experiments with a half-life of 35 min under noon summertime sunlight at 45° latitude, while cimetidine was shown to be resistant to direct photolysis. The results of these experiments, combined with the expected steady state near-surface concentrations of single oxygen and hydroxyl radicals, indicate that photo-oxidation mediated by single oxygen radicals is the likely degradation pathway for cimetidine in most natural waters, while photodegradation by direct photolysis is expected to be the major pathway for ranitidine. However, the extent of photo-induced degradation of pharmaceuticals can vary significantly for different pharmaceuticals, and it strongly depends on the aqueous constituents (such as humic and fulvic acids) present in solution. In addition, light levels within STPs are likely to be much lower than in the environment (effectively zero), due to the higher solids content. Indeed, Koutsouba et al. found diclofenac to be widespread in Greek domestic sewage effluent, with concentrations in effluent ranging from 10 to 365 ngL^sup −1^. Given the inherent photosensitivity of this compound, its presence in sewage effluent would seem to indicate that photodegradation is highly unlikely to take place within STPs where light penetration is minimal at best.”
The above demonstrates that it may be possible to remove pharmaceuticals and other organic contaminants from sewage using available technologies. However, these methods traditionally require significant costly upgrades to the STPs. Capital and operational costs are especially high for ozonation plants and other tertiary treatment options.
The pre-treatment method described below provides an inexpensive chemical installation and method to degrade and remove a number of chemicals/pharmaceuticals in wastewater influent via acid reduction and removal of suspended solids of sorbed polar pharmaceuticals/chemicals before liming the filtered acidified influent to further precipitate heavy metals and other chemicals/pharmaceuticals and raise the pH of the treated effluent for biodigestion with anaerobic and aerobic bacteria to further remove chemicals/pharmaceuticals.